>>1976889update
i think i found the issue
it wasnt arm pump
i think it was the angle i set my brake levers at
i have them very low like im used to on my mtb but ofc thats better for riding stood up
i think when sat down my wrists are angled down too much and stretching my tendons out
after raising them slightly it seems to have gone away
if it starts happening again im just gonna start stretching my hands and arms out more frequently before and during the flat parts of my rides